The present invention relates to a novel sheet deliverer for rotary printing machines having gripper members carried by continuous chains and braking rollers acting on the underside of the sheet positioned directly upstream of a delivery pile. Upstream of the delivery pile and braking rollers, there are arranged blower nozzles which direct an airstream of adjustable velocity against the underside of the sheet being guided by the gripper members in a direction opposed to the sheet delivery direction.
A conventional sheet deliverer of the type roughly described above has been proposed in DE-AS No. 1,148,241. Directly ahead of the braking rollers, a blast pipe having several blower nozzles extends across the sheet deliverer below the chain path. An airstream is delivered from the blower nozzles in a direction opposed to the sheet delivery direction and is directed at a very acute angle against the underside of the approaching sheet. The airstream leaving the blower nozzles causes a drop in the static pressure on the underside of the sheet and creates an air cushion between the sheet guide table and the sheets transported by the chain grippers. The reduction of static pressure on the underside of the sheet causes reliable contact between the sheet underside and the braking rollers. However, the sheet underside slides over the blower nozzle pipe and in the case of double-sided sheet printing, such as in the perfecting process, smearing of the fresh print on the sheet underside is an unavoidable consequence.
Another conventional sheet deliverer, described in DE-AS No. 2,135,105, has braking nozzle strips arranged upstream of the delivery pile, extending across the sheet width and directed against the sheet movement approximately parallel to the sheet path. On the approach side of the sheet, the braking nozzle strips have a horn-shaped bent guide plate under the sheet conveyor line, with lower part being bent downwards and arranged widthwise. The air leaving the braking nozzle strips acts on the underside of the approaching sheet drawing it down onto the braking nozzle strips. Due to the powerful braking effect, secondary grippers are provided which pull the sheet over and off the braking nozzle strips after the leading sheet edge has been released by the gripper members carried by the chain deliverer. With such a conventional sheet deliverer, the sheet is retarded and tensed when deposited on the top of the delivery pile. However, a disadvantage here again is that the braking nozzle strips tend to smear print on the sheet underside. The sheet deliverer according to DE-AS No. 2,135,105 is, therefore, only suitable for use on rotary printing machines which operate on the principle of face printing.
Another conventional configuration of a sheet deliverer is shown by DE-AS No. 2,544,566. The path of the delivery chain is designed with an inclined rising section and a horizontal section extending over the delivery pile. A sheet guide plate is arranged under the inclined rising section and part of the horizontal section of the delivery chain to produce an uninterrupted air cushion extending to the braking rollers. This type of conventional sheet deliverer is of a two-section design with the front edge of the sheet guide plate section, which is nearest to the delivery pile, securely mounted to the braking roller. When making format settings on the braking roller, the adjustable part of the sheet guide plate is also set.
As already mentioned above, such a conventional type of sheet deliverer provides an air cushion above the sheet guide surface extending as far as the braking rollers. However, even with such a conventional sheet deliverer, it is not possible to fully exclude the possibility of blotting the fluttering sheet. Thus, in order to ensure that the back section of the transported sheet also contacts the braking rollers, it is necessary, at least at high sheet conveying speeds--for example 8000 sheets per hour--to provide blowers above both the rising and the horizontal section of the delivery chain to ensure that the transported sheet is pressed onto the braking rollers. However, even with such a blower arrangement, the sheet may be passed onto the sheet guide plate and, thus, smeared.
Often, the blowers must powerfully operate at high velocities to press the back sheet end down. The air supplied by these blowers will then also enter the space between the next approaching sheet and the sheet guide plate. The air cushion generated by the sheet conveying speed is increased considerably by this occurrence, leading in turn to the sheet being pressed up or at least to fluttering of the sheet end at certain sheet conveying speeds. Such disturbed sheet feeding requires a further increase in the blower power, to ensure that the sheet end is pressed down. However, this results in even more air entering under the next approaching sheet. The consequences of such occurrences is that the power of the blowers blowing onto the topside of the sheet as well as the conveying power of the chain deliverer must be reduced considerably to return to a state of reliable delivery.
The object of the present invention, however is to avoid the disadvantages of the above-mentioned conventional sheet deliverers. It is a primary object of the present invention to guarantee that, for all sheet conveying speeds, the back end of the sheet guided by the gripper members of a chain deliverer is reliably drawn onto the braking rollers and that it is possible to set the entire sheet braking and guidance system to act on print-free parts of, for example, a double-printed sheet. The guide yoke blowers provided at the upper end of the sheet guide plate can be adjustable concurrently with the braking rollers to ensure that only print-free margins are contacted.
The output of air from the blower nozzles at the lower end of the guide yoke blowers has basically two effects. First, the static partial vacuum created causes the print-free portions of the sheet to contact the guide yokes of the guide yoke blowers. This responsively causes the sheet to be tensed. Second, in the remaining surrounding area, the air streaming out of the blower nozzles increases the air cushion between the sheet guide plate and the underside of the sheet. Therefore, whereas firm contact of the back sheet section on the braking rollers is achieved, the controlled increase in the air cushion also provides an assurance that the images on the underside of the sheet are not smeared at the upper end of the sheet guide plate. The above sheets are also important since the sheet end has passed the blower nozzles when the leading edge of the sheet is released by the gripper members of the chain deliverer. This allows retardation to be set in such a way that the leading edge only slides against the stops of the pile at low velocity.
A further embodiment of the subject invention is the provision of tensioned guide bands located between the braking rollers and the guide yoke blowers and which are adjustable with respect to the braking rollers. Alternatively, a flexible guide cloth can be similarly provided. A further embodiment of the present invention is the provision of easily interchangeable cover plates between adjacent braking rollers which are replaceable with different-sized cover plates when the braking rollers are laterally adjusted.